Handle stringified JSON tool call arguments

[kuosuko]

Fixes #169.

Problem

JSONToolCallParser assumes arguments is always a JSON object, but some models emit it as a JSON-encoded string instead.

That means payloads like this fail to parse today:

{"name":"get_weather","arguments":"{\"location\":\"Paris\"}"}

As a result, Swift drops the tool call entirely.

Fix

Normalize the outer JSON payload before decoding ToolCall.Function:

• decode the top-level object with JSONSerialization
• if arguments is a string, parse that string as JSON
• replace arguments with the parsed value
• decode the normalized payload using the existing JSONDecoder

This preserves the current behavior for object-shaped arguments while adding compatibility for stringified arguments.

Testing

• swift test --filter ToolTests
• added regression tests for stringified object arguments
• added regression tests for stringified empty arguments
• added regression tests for stringified array arguments

I also verified locally with Granite tool calling that the original parser emitted no parsed tool call while the patched parser successfully emitted get_weather(location: "Tokyo", unit: "celsius").